High precision membrane osmometry has demonstrated that histones in 2 M NaCl exist as a dissociating complex of octamers, hexamers and dimers; no evidence for the existence of a heterotypic tetramer is found. The stabilization of DNA in nucleosomes by interactions with histones is almost totally entropic in origin, consistent with the primary forces being electrostatic interactions. Under proper conditions, we demonstrate that H1 has two effects on reconstituted chromatin made with the four inner histones. First, it appears to alter the repeat length determined by micrococcal nuclease digestion from about 150 base pairs to over 180 base pairs, similar to native material. Second, it appears to alter the wrapping of DNA around the octamer such that the linking number change per nucleosome is altered from -1 to -2. NMR studies of 145 base pair poly(dG-dC) alone and in core particles show that the alternating conformation characteristic of the high salt form Z-DNA is not present when the polymer is associated with these basic proteins.